1. Field of the Invention
The present invention relates to a method for controlling a wire-cut electric discharge machine, and more particularly, to a control method for machining a corner of the workpiece.
2. Description of the Related Art
A wire-cut electric discharge machine generates an electric discharge between its wire electrode and a workpiece, and cuts the workpiece into a desired shape by shifting the wire electrode with respect to the workpiece according to a machining command data.
As shown in FIG. 1 (PRIOR ART), when the wire electrode 1 moves in a predetermined direction (i.e. a right direction in FIG. 1) for forming a slit 3 on a workpiece 2, a pressure is usually generated between the wire electrode 1 and the workpiece 2 due to electric discharge, thereby causing the wire electrode 1 to be bent backward or in a direction of the arrow S, i.e. a direction opposite to the direction of movement. For this reason, the wire electrode 1 is bent or curved backward with respect to a straight line connecting a pair of wire guides 4 and 4.
In the case of a straight-line machining operation, this deflection will not substantially affect the machining accuracy for the worse. However, in order to form a corner, the electrode 1 needs to change its direction of movement in accordance with a machining command, for example, in a normal direction as shown in FIG. 2a (PRIOR ART). In such a corner formation, the slit 3 having a profile as indicated by a solid line 3b is not formed. An electric discharging portion of the wire electrode 1 is drawn towards an inside of the corner because of the above deflection of the wife electrode 1, to cause a discrepancy from the desired profile, as indicated by a dotted line 3a. An amount of such discrepancy is given as .delta..
Furthermore, as shown in FIG. 2b (PRIOR ART), a similar discrepancy will occur in an arc-shaped corner in the case where a straight-line machining operation is followed by an arc machining operation.
It is known that the problem of discrepancy, such as one occurring when a straight-line machining operation is followed by another straight-line machining operation differing in the direction, or when a straight-line machining operation transfers to an arc machining operation, as is discussed in the foregoing, can be solved to a certain extent by changing the conditions of an electric discharge at the corner. For example, the on-off interval of pulse current supplied can be varied during the corner machining operation so that an electric discharge energy supplied between the wire electrode and the workpiece can be reduced to reduce the deflection of wire and resulting discrepancy of the corner.
Moreover, in wire-cut electric discharge machining, a highly-pressurized machining fluid is usually jetted against the wire electrode in the machined slit from upper and lower sides of the workpiece to remove machining chips and cool down an electrically heated portion. Hence, this machining fluid becomes another cause of the deflection of the wire electrode in the corner machining operation, thereby weakening the above-described corner discrepancy preventive effect due to the control of the discharge energy.
SUMMARY OF INVENTION
According to the present invention, a specified section is designated in a corner of a portion to be machined when the corner has an arc shape or formed with two straight lines intersecting with each other. A pressure and/or an amount of the machining fluid is controlled to be lower than those of an ordinary straight-line machining operation only within this specified section so that deflection amount of the wire electrode can be suppressed to or below a predetermined value when the wire electrode is moving within the specified section.
Furthermore, in addition to the pressure and/or amount of the machining fluid, an electric discharge energy supplied between the wire electrode and the workpiece is controlled to be lower than those in an ordinary straight-line machining operation only within this specified section.
The specified section is designated on the bases of the shape and size of the corner. Furthermore, the degree of adjustment of the machining fluid pressure and/or supply amount, as well as the same of electric discharge energy supplied, are determined in accordance with the shape and size of the corner.